The operation of electronically timed detonators, also known as electronic delay detonators, or EDDs, for blasting, mining, quarrying and similar operations is conventionally performed by use of a network or harness of wires that connect all the detonators together, and the devices that control them. Typically, each detonator is located below ground associated with a mass bulk of explosive material, with a connection made to the aforesaid harness at the top of the hole which contains the explosive.
This surface harness wire network must be connected to the detonators at the blast site to other components such as blasting machines. This process causes significant labour costs and generates many of the faults that occur due to failed or damaged connections. Moreover, the wire itself becomes a nuisance. Firstly it prevents easy movement of men and vehicles over the blasting site and is itself easily damaged. Secondly it has to be gathered for disposal being unfit for reuse or it becomes an undesirable material contaminant of the ore body being extracted.
It is therefore desirable to eliminate the surface wiring for EDDs and control the detonators remotely using some wireless means of communication. EDDs to be effective and safe preferably have two-way communication with the controlling device in direct communication with the detonators, also known as the blasting machine. Often, the communication means must therefore provide reliable transfer of messages, from a blasting machine to a large number of EDDs. The physical circumstances, particularly in open cast mining or quarrying, give rise to EDDs being laid out in patterns that can extend several hundreds of meters over somewhat irregular terrain.
Persons of skill in the art recognize the potential of wireless detonator systems for significant improvements in safety at the blast site. By avoiding the use of “wired” physical connections (e.g. electrical wires, shock tubes, LEDC, or optical cables) between detonators, and other components at the blast site (e.g. blasting machines) the possibility of improper set-up of the blasting arrangement is reduced. With traditional, “wired” blasting arrangements, significant skill and care is required by a blasting operator to establish proper connections between the wires and the components of the blasting arrangement. In addition, significant care is required to ensure that the wires lead from the explosive charge (and an associated detonator) to a blasting machine without disruption, snagging, damage or other interference that could prevent proper control and operation of the detonator via the attached blasting machine. Wireless blasting systems offer the hope of circumventing these problems.
Another advantage of wireless blasting systems relates to facilitation of automated establishment of the explosive charges and associated detonators at the blast site. This may include for example automated detonator loading in boreholes, and automated association of a corresponding detonator with each explosive charge. Automated establishment of an array of explosive charges and detonators at a blast site, for example by employing robotic systems, would provide dramatic improvements in blast site safety since blast operators would be able to set up the blasting array from entirely remote locations. However, such systems present formidable technological challenges, many of which remain unresolved. One obstacle to automation is the difficulty of robotic manipulation and handling of detonators at the blast site, particularly where the detonators require tieing-in or other forms of hook up to electrical wires, shock tubes or the like. Wireless detonators and corresponding wireless detonator systems will help to circumvent such difficulties, and are clearly more amenable to application with automated mining operations. In addition, manual set up and tieing in of detonators via physical connections is very labour intensive, requiring significant time of blast operator time. In contrast, automated blasting systems are significantly less labour intensive, since much of the set-up procedure involves robotic systems rather than blast operator's time.
Progress has been made in the development wireless detonator assemblies, and wireless blasting systems that are suitable for use in mining operations, including detonators and systems that are amenable to automated set-up at the blast site. Nonetheless, existing wireless blasting systems still present significant operational concerns, and improvements are required if wireless systems are to become a viable alternative to traditional “wired” blasting systems. These concerns include, but are not limited to, calibration of detonators for a timed blasting event. An array of detonators at a blast site may include several, perhaps hundreds, of EDDs, and each may be individually programmed with a carefully selected delay time. At a time of blasting, a blasting machine (or machines) associated with the detonators may transmit to the detonators a command signal to FIRE upon which time the detonators count down their respective, pre-programmed delay times. For selected EDDs, such delay times may be programmed with an accuracy of lms or sometimes even greater.
Typically, each EDD at a blast site may have its own internal (or otherwise individually associated) clock to countdown its programmed delay time. To account for variance in clock accuracy either between individual detonator clocks, or for each detonator clock over a period of time, detonator clocks are generally calibrated at the blast site just prior to detonator initiation, for example by checking the rate of oscillation of each detonator clock against a standard (i.e. “master”) clock. For example, each EDD may have transmitted thereto a calibration-count-start signal and a calibration-count-stop signal, wherein the start and stop signals are separated by a fixed, known time interval. For example, if the start and stop signals transmitted by a master clock are 1024 ms apart, each detonator can record its own clock count for the intervening 1024 ms period between the receipt of the two signals, and this clock count is then used (either by the detonator or more commonly by an associated blasting machine) to establish its accuracy relative to the master clock. Subsequently, each clock count of each detonator may be adjusted to count down its programmed delay time with compensation for any inaccuracy in its internal clock.
Such calibration techniques are more particularly useful for shorter delay times. However, detonator clock speeds may vary somewhat over time, and clocks may drift relative to one another if such variances remain unchecked. For example, each detonator will have its own internal capacitor, with current draw and voltage characteristics that will affect clock operation over time. Thus, when longer delay times are employed clock accuracy may deteriorate even after calibration of detonator clocks in accordance with the methods discussed above. This applies not only to blasting systems that employ a surface harness wired network, but also applies more particularly to wireless detonator systems involving wireless detonator assemblies, which must be individually powered by an internal power supply, the latter, inevitably, giving another source of variation in the system. It follows that improvements are required in methods and apparatuses for detonator clock calibration.